1. Field of the Invention
The present invention relates to a semiconductor laser module with an electronic cooling device, and more particularly to a semiconductor laser module of a direct modulation system in which a high-frequency input signal is modulated directly into an optical signal.
2. Description of the Prior Art
In recent years, the semiconductor laser module has been under energetic attempts for applications to such communication fields as CATV and public communication that deal with signals of microwave/semi-microwave frequency range, giga bit high-speed digital signals, and other high-speed modulation signals, so that the semiconductor laser module has begun to be put into practical use. Particularly for short-and middle-distance transmission ranging from several hundreds of meters to several kilometers involving less accumulation of noise and signal distortions, the direct intensity modulation system in which a high-frequency signal is modulated directly into an optical signal as a modulation signal is suitable. In such a semiconductor laser module, oscillation characteristics of the semiconductor laser have a large temperature dependency, so that shifts of threshold current density or oscillation wavelength may take place depending on temperature variation. Therefore, in order to obtain a stable laser oscillation, it is essential to maintain laser diodes at a constant temperature. Thus, cooling devices such as a Peltier-element electronic cooling device is used to control the temperature of the laser diodes.
In prior-art semiconductor laser modules, module components such as the electronic cooling device and the ground line form a resonance circuit. For example, as shown in FIG. 20, the circuit resonates with an input modulation signal in a frequency band around 1.6 GHz, causing in some cases a phenomenon that the optical frequency response level would lower.
For this reason, in public communications in which frequencies close to the aforementioned frequency band such as 1.5 GHz band or 1.9 GHz band are frequently used, or in high-speed digital communications in which transmission rates close to the aforementioned frequency band are used, it has been difficult for such semiconductor laser modules to ensure a sufficient modulation signal.